1. Field of the Invention
The present invention relates to a printing apparatus and power supply control method. More particularly, this invention relates to a printing apparatus having a printhead which prints according to the inkjet method, and a power supply control method applied to the apparatus.
2. Description of the Related Art
As a conventional printing apparatus, there are known a thermal transfer printing apparatus, and an inkjet printing apparatus which prints a text, image, or the like by discharging ink onto a printing medium such as a printing sheet. The inkjet printing apparatus used as information output means (e.g., a printer, copying machine, or facsimile machine) discharges ink to print while moving the relative positions of a printing medium and inkjet printhead (to be referred to as a printhead hereinafter). The image quality of inkjet printing is determined by control of the relative speeds of the printhead and printing medium, control of the discharge timing in accordance with the relative speed control, stability of power supply to the printhead, and the like.
Inkjet printing apparatuses are generally classified into a so-called serial type and full-line type in accordance with the form of the printhead used. Of these two types, the serial type inkjet printing apparatus discharges ink to print while moving the printhead, and is widely adopted.
Some printheads for discharging ink discharge ink by the operation of a piezoelectric element, while some printheads discharge ink by creating instantaneous film boiling of ink. The printhead for boiling and discharging ink supplies discharge energy by boiling ink near a heater using thermal energy which is generated by energizing the heater arranged near an ink channel near an ink orifice.
In order to maintain high image quality, it is important to always stably supply energy for discharging ink, discharge ink under the same conditions, and thereby obtain uniform ink droplets. In actual printing operation, however, the duty ratio changes depending on image data, and the number of simultaneously energized heaters varies. The printhead driving conditions change under the influence of voltage fluctuations caused by the output current difference of the power supply unit, the difference in voltage drop caused by the resistance of the power supply line, and the like.
Conventionally, ink discharge from the printhead is so executed as to satisfy stable discharge conditions by, e.g., precisely controlling the power supply output voltage and configuring the power supply line so as to minimize the electric energy loss.
As a means for reducing a voltage drop caused by the resistance of the power supply line and stabilizing the power supply voltage, for example, Japanese Patent Publication Laid-Open No. 2003-211671 proposes a printing apparatus in which a DC/DC converter is mounted on a circuit board on a carriage unit supporting a printhead.
This arrangement greatly reduces a power supply voltage drop amount generated by the driving current of the printhead. To perform constant-voltage control, the DC/DC converter described in Japanese Patent Publication Laid-Open No. 2003-211671 executes voltage feedback control of detecting an output voltage, comparing it with the reference voltage, and regulating the switching time ratio. For this reason, the feedback circuit suffers a response delay. When the load current abruptly varies, the control circuit cannot follow such a variation. Thus, the output voltage drops upon an abrupt increase in load current and rises upon an abrupt decrease in load current.
This will be explained with reference to FIG. 11.
FIG. 11 is a timing chart showing changes in output voltage and printhead current, which occur in accordance with an image signal transferred to the printhead and a latch signal representing one cycle of printing operation.
To print an image of repetitive thick lines and blank lines, like a stripe pattern, image signals representing thick lines and blank lines are alternately transferred every cycle, and the head current abruptly changes every cycle, as shown in FIG. 11. A heater driving voltage (VH) applied to the printhead is stable at about 20 V in the steady state, and drops by ΔV1 at the time interval until feedback control of the power supply circuit responds to an abrupt increase in head current. In addition, the voltage rises by ΔV2 at the time interval until feedback control of the power supply circuit responds to an abrupt decrease in head current. Hence, as shown FIG. 11, the fluctuation width of the output voltage is ΔV1+ΔV2=ΔV. Unless the voltage fluctuation width (ΔV) is within a predefined value (tolerable fluctuation range of the power supply voltage) for stabilizing driving of the printhead, an ink discharge failure occurs or the service life of the printhead shortens.
In order to suppress the fluctuation width, the capacity of the output capacitor of the DC/DC converter may be increased. However, downsizing is demanded of the DC/DC converter arranged at a movable portion such as the carriage, in addition to the printhead. It is not preferable in terms of size and cost to use a large-capacity capacitor.
In order to meet this requirement, there is proposed, e.g., a technique disclosed in Japanese Patent Publication Laid-Open No. 2003-225997. Japanese Patent Publication Laid-Open No. 2003-225997 discloses a technique of counting image signals transferred to the printhead, determining the magnitude of the head current, correcting the feedback control signal of the DC/DC converter, and thereby improving the response of the power supply upon an abrupt change in head current. By using this technique fluctuations of the power supply voltage are suppressed.
In the prior art, however, the correction signal is added to the feedback circuit of the power supply circuit, complicating the design of the power control circuit. Although response characteristics improve, the stability of the feedback circuit may be impaired in the steady load state.
Thus, only a designer having the know-how to design a power supply circuit can design and develop a highly reliable power supply circuit. Further, a correction signal processing circuit and an addition circuit to a feedback signal are also necessary, increasing the product cost.